I have a base Mac Pro 4,1 (GT120) with an Apple 24" display plugged into the Mini-DP and an old HP lcd plugged into the DVI port. Every couple of minutes, the Apple display will flicker, or flash black for like half a second. The HP display doesn't flicker at all. I've made sure the machine has all the software updates from Apple and have even installed drivers for the GT120 from nVidia. I reset the PRAM yesterday and the problem persists today. Any ideas?